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I've searched for past responses but found nothing regarding my issue. I've recently upped my mileage and now a week later (for the past few days) my right knee feels unstable and loose. There is a dull achy feeling but nothing unbearable. I can bend the knee and even stand on the knee one legged, but there are times when I feel like it wants to give out. I'm 25 y/o and currently in the middle of marathon training. I can run on it, but I'm scared it will get worse. Anybody experience this issue before?

Strength exercises. Try doing step-ups and once you can do like 100 of them add in step-downs too. They strengthen the inner quad which helps stabilize your knee. Do them on both legs even though only one is bothering you.

Thank you! I'll add that doing step-ups/stairs is one of the times when it feels like it's going to give out. I'm guessing from weakness? I'm just fearful of something being torn and not knowing about it until it's too late. Considering I've never torn anything before (that I know of) it's had me worried. Should I continue to run?

Probably weakness. Don't go overboard with the step-ups. Start with a really small number and make sure it doesn't make it worse. Then gradually build up once or twice a week. Keep doing them even when you're healthy again. If you get lazy then this could come back.

I can usually run through minor knee issues without a problem but that's your call. I say try ice and strength stuff for a little and if it doesn't get any better after doing that consistently, or if it starts to get worse, then take some time off. But if you're really nervous then a day or two of cross-training as a precaution probably won't hurt you much.

If your knee feels like it wants to buckle or give way it could be because of poor quads control. Try strengthening exercises like pistols focussing really hard on keeping your knee inline with your foot. Only go as low as you can keep control.
If you feel your knee locking or clicking a lot it could be a sign of some miniscal or other wear & tear inside and you should get it assessed properly.

Thanks! I've only been running seriously for over a year now and I'm always worried about internal damage in the joint. There's not much popping and cracking going on so hopefully strengthening my leg will solve the issue!

My left knee is the loose one. Doctor told me to do leg extensions with a little bit of weight to strengthen the quad. Doing 3 sets of 30 with about 25lbs. I do my whole routine twice including that one.
Also, while just sitting at your desk, computer or whatever.. practice leg extensions, slow and make sure to hold the extension for a sec or two before letting it back down. It's something.
Thanks for the steps advice - I'm going to work that in tomorrow.

Still too early to tell if this is going to work but it makes a lot of sense.

I never really figured that I would need to work on my quads being that I run 35-40 miles a week, but now that I think about it, I hardly use my upper legs as much as I do my lower legs when running. See, I'm a bigger guy (225 lbs) that runs at a steady 10:00 min mile pace so my stride is hardly long enough to force my quads to get a workout. From now on, I'm working on my upper legs as much as possible. This could be a serious issue if it gets any worse!